<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
/* 针对宽度小于 400 像素的设备： */
body {
  background-repeat: no-repeat;
  background-image: url('../static/img/tulip_peach_blossom_s.jpg'); 
}

/* 针对宽度大于等于 400 像素的设备： */
@media only screen and (min-width: 800px) {
  body { 
     background-image: url('../static/img/flower-5.jpg'); 
  }
}
</style>
</head>
<body>

<p style="margin-top:660px;">请调整浏览器宽度，背景图像会在 400 像素（这个断点）发生变化。</p>

</body>
</html>
